Hitherto, ultrahigh-molecular-weight polyolefin, particularly ultrahigh-molecular-weight polyethylene is excellent in impact resistance, abrasion resistance, sliding properties, and chemical resistance as compared with commonly used polyethylene, and can be used as sliding parts. Furthermore, since the polyolefin has a small friction coefficient, it is used in skis and the like. However, since the ultrahigh-molecular-weight polyethylene usually has a high crystallinity and shows a white opaque appearance, it is inferior in transparency even when it is molded into a thin sheet or film and thus designing ability for brand names and the like on soles of skis is impaired, so that a material excellent in transparency is desired. For the purpose of improving transparency thereof, Patent Document 1 proposes an ultrahigh-molecular-weight ethylene copolymer obtained from ethylene and another α-olefin.
Since ultrahigh-molecular-weight polyethylene has a high molecular weight and a bad flowability when melted, it is difficult to mold it into a pellet form in production sites unlike the case of usual polyethylene and hence it has been shipped in a powder form. Therefore, the ultrahigh-molecular-weight polyethylene in a powder form has been used at its molding.
In molding sites, bulk density of a powder is extremely important. For example, an ultrahigh-molecular-weight molded article for skis has been produced by filling a cylindrical mold with the ultrahigh-molecular-weight polyethylene powder and subjecting it to compression molding, followed by scraping the molded article thinly in the circumferential direction. At this time, in the case that the bulk density of the ultrahigh-molecular-weight polyethylene powder is low, there arise problems that a sufficient weight of the ultrahigh-molecular-weight polyethylene powder for the molded article does not enter into the mold at its filling and bubbles are apt to be present in the molded article. For solving the problems, there may be considered a method of using a mold having a larger volume and extending the molding time but there arises a problem that electric power to be consumed remarkably increases because of increased heat capacity and heat radiation of the mold. In addition, since the method requires increased working distance and working time of a compressor, molding efficiency remarkably decreases.
On the other hand, also in the production of the ultrahigh-molecular-weight polyethylene powder, the bulk density of the ultrahigh-molecular-weight polyethylene powder formed is very important. Since it is possible to increase weight of the ultrahigh-molecular-weight polyethylene per unit volume contained in a polymerization vessel by increasing the bulk density, concentration of the ultrahigh-molecular-weight polyethylene powder in the polymerization vessel can be enhanced. Thereby, it is possible to enhance productivity of the ultrahigh-molecular-weight polyethylene even in the same process. Moreover, in the case of transporting and storing the ultrahigh-molecular-weight polyethylene powder discharged from the polymerization vessel, the bulk density is important. A higher bulk density may decrease clogging at its transportation and it is possible to increase a storable amount per unit volume.
It is generally known that the bulk density of the ultrahigh-molecular-weight polyethylene powder is increased by adding an additive, e.g., a lubricant such as calcium stearate. On the other hand, it is also known that the additive deteriorates thermal fusibility at molding and the additive also contaminates the surface of the molded article because of its bleeding onto the surface. For the reasons, it is desired to enhance the productivity of the ultrahigh-molecular-weight polyethylene powder and to increase the bulk density in the state where no additive is present.
As above, the bulk density of the ultrahigh-molecular-weight polyethylene powder is one of very important properties necessary for ultrahigh-molecular-weight polyethylene. However, in Patent Document 1, the extremely important bulk density of the ultrahigh-molecular-weight polyethylene powder has not been investigated.
[Patent Document 1] JP-B-5-86803